There are various known techniques of preparing benzaldehydes. Miller et al., Journal of the American Chemical Society, Vol. 78, pp. 1017-1034 (1956) and Patai, The Chemistry of the Carbonyl Group, Interscience Publishers, pp. 125-127 (1966) show that benzaldehyde is one of the products obtained by the oxidation of styrene. U.S. Pat. No. 3,435,061 (Grasselli et al.) suggest that the poor benzaldehyde yields of Miller et al. and Patai could be improved by conducting the oxidation in the presence of certain catalysts, such as mixed metal oxide catalysts, at 300.degree.-800.degree. C. European Patent Application No. 0 012 939 (Nishizawa et al.) shows that p-cresols can be oxidized to 4-hydroxybenzaldehydes in the presence of at least one equivalent of base and a catalytic amount of cobalt or a cobalt compound. U.S. Pat. No. 4,009,210 (Cahoy) teaches that 3,5-dihydrocarbyl-4-hydroxybenzaldehydes can be prepared by reacting a 2,6-dihydrocarbylphenol with hexamethylenetetramine or a combination of formaldehyde and ammonium acetate in aqueous acetic acid.